The present invention relates to a method of calibrating a scanning system. A scanning system in this specification should be understood to mean a combination of a machine and a probe which together are capable of use in scanning an artefact in order to obtain information about its size, shape or surface contours. The machine may be a co-ordinate measuring machine (CMM), or robot, and the probe is an analogue probe which may have a workpiece-contacting stylus or may be a non-contact probe. The machine has measuring devices for measuring the movement of the machine parts in three nominally orthogonal directions (referred to as X,Y and Z axes), and the probe includes measuring transducers for producing outputs indicative of the is deflection of the probe in three nominally orthogonal directions (referred to as the a,b and c axes).
In general terms the present invention relates to a method of calibrating a scanning system dynamically, whereby errors in the system produced when scanning an artefact at different scanning speeds (and hence at difference accelerations) may be mapped, without the need to calibrate the probe itself.
Methods of correcting machines for acceleration induced errors are known.
One example of such a method is described in European Patent No. 318557. In this method a first article from a batch of nominally identical articles is scanned at a relatively slow speed, noting the measurements of the positions of a number of datum points on the article. The scanning operation is repeated at a relatively fast speed noting the measurements of the positions of the same points. Any difference in the measurements are noted as errors in a correction table.
Thereafter all of the articles are scanned at the relatively fast speed taking measurements of the positions of corresponding points on each article, and these measurements are corrected for machine accelerations using the previously noted errors.
This method requires the probe to have been accurately calibrated before the measurements are taken, and does not account for machine errors other than dynamic deflections.
Another example of such a method is described in U.S. Pat. No. 5,594,668. This patent discloses scanning a ring gauge at different velocities, and hence at different accelerations of the machine slides, and determining the differences in the measured X,Y values of a plurality of datum points as a function of the acceleration components of the machine in the X and Y directions. These measurements are repeated with the ring gauge positioned at several different places in the machine""s working envelope, and a set of correction data is stored for subsequent correction of the measurements of workpieces.
This method produces a map of corrections based on an accurately known gauge of symmetrical form, but does not necessarily produce a result which is applicable to a non-symmetrical workpiece, and does not take account of surface finish, or of different materials.
We have found that when using a probe having a workpiece contacting stylus, stylus slippage on the surface being used for the calibration can be a source of significant errors in the measurements of the positions of the points on the surface which are used as the datum points, leading to errors in the calibration/correction data.
The errors occur because the simple correlation assumed between the machine displacement and probe stylus deflection is destroyed by stylus slippage.
Stylus slippage occurs when, for whatever reason, the probing force component in a direction in the plane of the surface contacted would otherwise exceed the product of the force component in the direction normal to the surface and the effective friction coefficient. Such a situation can arise from one or more reasons, e.g. the commanded machine direction may not be exactly normal to the contacted surface, and/or machine inaccuracies may lead to the machine not travelling accurately in the commanded direction, and/or probe inaccuracies (or design) may cause the probing force direction to differ from the probe deflection direction.
The present invention provides a method of calibrating a scanning system in which the effects of stylus slippage caused by probe and/or machine inaccuracies are minimised.
According to one aspect of the present invention there is provided a method of calibrating a scanning system comprising the following steps:
a) moving the probe stylus towards a surface of an artefact in a direction which is nominally normal to the surface, and contacting the surface at a number (N) of specific datum points on the surface,
b) using only the components of machine movements and probe stylus deflections which are normal to the surface at the points of contact therewith, making a determination of the positions of each of the datum points at the instant that the stylus tip is just in contact with the surface,
c) scanning the surface of the artefact at a predetermined finite probe stylus deflection and at a plurality of different speeds each time nominally passing through the datum points several times,
d) using the components of machine movements and probe stylus deflections which are normal to the surface, making further determinations of the apparent positions of each of the (N) datum points, and recording any differences in the normal direction from the positions determined in step b) for each speed,
e) from the differences recorded in step d) identifying the highest scanning speed at which the variations in the measurements of the positions of the datum points noted during each scan remain within a predetermined tolerance,
f) storing the identified speed and the differences in the measurement at that speed.
The method according to the invention is based on two theories. The first is that there can be no stylus slippage in the directions normal to the surface of the artefact. All stylus slippage must be parallel to the surface. The second is that the above-mentioned probe inaccuracies become negligible when the probe deflection is zero.
Thus by utilising only the components of machine movement and stylus deflection which are normal to the surface and determining these values when the stylus is just in contact with the surface, but is not deflected, the resulting measurements of the datum points are free of probe errors, and free of errors due to stylus slippage.
The determination of the position of each datum point at the instant the stylus tip is just in contact with the surface of the artefact may be achieved by driving the probe into the surface and synchronously recording the components of machine movements and probe deflections which are normal to the surface until the probe deflection reaches a predetermined limit. The recorded values are then extrapolated back to determine the position of the machine in the direction normal to the surface when the stylus was just in contact with the surface.
Alternatively, and preferably, the probe is driven into the surface until the stylus deflection reaches a predetermined limit and is then reversed at a known and controlled low velocity. During the reversal the components of machine movements and probe deflections normal to the surface are recorded synchronously until the stylus leaves the surface. The recorded values are then extrapolated to determine the position of the machine in the direction normal to the surface when the stylus just left the surface. This is effectively the same as the position when the stylus just contacted the surface.
During the scanning step, the outputs of the measuring transducers of the probe in the a,b, and c axes are transformed into incremental values of X, Y, and Z using a probe transformation matrix.
Once the maximum scanning speed has been established by this method a map of the errors in the direction normal to the surface at the (n) points can be stored along with the data relating to the scanning speed, the particular artefact or feature, the particular CMM and the part location and orientation on the CMM, the particular probe and stylus configuration, and the probe matrix and nominal probe deflection used.
Instead of storing this data in the machine computer, possibly along with many other error maps for other workpieces, in accordance with a novel feature of the invention, this data may be stored outside of the machine as part of, or in association with, the part program associated with a workpiece. A part program is the software program which is loaded into the computer of a measuring machine when a workpiece is to be measured, and which identifies to the measuring machine both the details of workpiece to be measured, and the moves to be accomplished by the machine in order to make the required measurements.
In order to avoid machine errors affecting the accuracy of the results it is preferable to error map the machine and qualify the stylus tip as to its diameter and position relative to the axis of the machine spindle.
Thus according to a further aspect of the present invention a method of dynamically calibrating a scanning system comprises the steps of:
a) error mapping the system statically,
b) determining the diameter of the stylus tip and its position relative to the probe using a datum sphere,
c) determining the positions of a plurality of datum points on a surface of an artefact with the probe stylus in contact with said surface and when at least the component of the stylus deflection normal to the surface is zero,
d) scanning the surface of the artefact passing through the datum points at a nominal stylus deflection and at the maximum speed at which the results are repeatable within a given tolerance,
e) subtracting the positions of the datum points determined with zero normal deflection of the stylus from the positions of the datum points produced during the scanning step to determine the measurement errors attributable to the scanning process in the direction normal to the surface at the nominal deflection,
f) storing the error values for subsequent correction of measurements taken on a similar artefact at the same speed and deflection.